Accelerometer and its fabrication technique

ABSTRACT

An accelerometer, comprises, a measurement mass, a top cap silicon wafer and a bottom cap silicon wafer, which both are coupled with the said measurement mass; the measurement mass comprises a support frame, a mass, and a plurality of resilient beams; the mass and the resilient beams are located within the support frame; the mass and the support frame are connected by several sets of the resilient beams, and each set comprises two resilient folding beams; the resilient folding beams are symmetrically provided with respect to the midline of the mass; a connection beam is provided in between each set of the resilient folding beams to connect the resilient folding beams together. Silicon wafers with electrodes are bonded on the top and bottom surfaces of the measurement mass; and forms a capacitor with the measurement mass. The accelerometer in the present invention has a large mode isolation ratio, and it is symmetrical in high order vibrational modes , which further decreases the noise of the MEMS chip. Therefore, the accelerometer has the advantages of high measurement accuracy, high stability, and low noise.

CROSS-REFERENCE

This application claims priority from Chinese Patent Application No.201210356542.3, filed Sep. 12, 2012 and entitled An Accelerometer andits Fabrication Technique.

TECHNICAL FIELD

This invention relates to a sensor, particularly to an accelerometer.

BACKGROUND

Nowadays, accelerometers have been used in various applications, suchas, measuring the magnitude of earthquake and gathering seismic data,detecting the magnitude of collision during a car collision, anddetecting the tilting direction and angle of a mobile phone or a gameconsole. As the micro-electro-mechanical systems (MEMS) technologycontinues to progress, many nano-scale accelerometers have been widelycommercially used.

There are two kinds of accelerometers which are commonly used:piezoresistive accelerometers and capacitive accelerometers.Piezoresistive accelerometer, such as Chinese invention patentapplication with Publication No. CN1748146 and Publication Date of Mar.15 2006, usually comprises mass and beams, piezo elements are providedon the beam. The mass moves according to the acceleration, and causesthe beam to deform, which also causes the change of resistance. However,under situations when there is no acceleration or the accelerationmagnitude is relatively small, the beam will not deform significantly.And there is no significant change in resistance. The accelerometer canonly detect acceleration when the magnitude of acceleration is largeenough to cause the deformation of the beam. Therefore, suchaccelerometer has low accuracy.

Capacitive accelerator, such as U.S. Pat. No. 6,805,008, withPublication Date of Oct. 19, 2004, also includes beam and mass. When theacceleration is present, the frame of the accelerometer will moveaccording to the acceleration direction, but due to inertia, there islittle displacement for the mass. Thus, the gap distance change betweenthe mass and another electrode causes a change in capacitance. Bothkinds of accelerometers are manufactured by micro fabrication techniqueand have the characteristics of small size and low manufacturing cost.However, the beams are resilient beams, and there are only four beamsconnecting the mass with the frame. Thus, when the outer frame moves,the displacement of each beam is relatively large. Also, each beam willnot create a uniform displacement and deformation, which leads tounsymmetrical vibrational modes.

SUMMARY

The present invention is intended to overcome the shortages of theexisting accelerometers, and to provide an accelerometer with relativelyhigh stability and reliability.

In accordance with the present invention to provide an accelerometer,comprising a measurement mass, a top cap silicon wafer and a bottom capsilicon wafer, which both are coupled with the said measurement mass;the measurement mass comprises a support frame, a mass, and a pluralityof resilient beams; the mass and the resilient beams are located withinthe support frame; the mass and the support frame are connected byseveral sets of the resilient beams, and each set comprises tworesilient folding beams; the resilient folding beams are symmetricallyprovided with respect to the midline of the mass; a connection beam isprovided between each set of the resilient folding beams to connect theresilient folding beams together.

The present invention also has the following features:

The connection beams are resilient connection beams.

The two ends of the resilient folding beams are located along the sameline.

One end of the resilient folding beam is respectively connected to thecorner of the mass.

The measurement mass has a double side silicon on insulator (SOI)structure, which includes top silicon layer, middle silicon layer, andbottom silicon layer; a silicon dioxide layer is provided between everytwo silicon layers. The double side silicon on insulator structure isalso referred as double side SOI structure.

A plurality of the resilient beams is respectively symmetrically formedin the top silicon layer and the bottom silicon layer to compose adouble layer structure.

Electrodes are respectively provided on the measurement mass, the topcap silicon wafer and the bottom cap silicon wafer.

In accordance with the present invention to provide a fabricationtechnique for the accelerometer, including the following steps:

Step 1, use photolithography, deep etching and etching to form aplurality of holes penetrating from the top silicon layer and bottomsilicon layer to the middle silicon layer of the double side SOI siliconwafer;

Step 2, deposit polycrystalline silicon in the holes to fill up theholes in order to form an electric circuit; then grow a silicon dioxidelayer on the surfaces of the top silicon layer and bottom silicon layerof the double side SOI silicon wafer, and polish the surface;

Step 3, use photolithography, deep etching and etching to form severalsets of symmetrical resilient folding beams on the double side SOIsilicon wafer; then use thermal oxidation to grow silicon dioxide on theexposed surfaces of the resilient folding beams, or use chemical vapordeposition (CVD) method to dispose a layer of silicon dioxide on theexposed surfaces of the resilient folding beams;

Step 4, use lithography and etching to remove the exposed silicondioxide in the middle silicon layer, and deep etch the middle siliconlayer to a certain depth;

Step 5, simultaneously perform corrosion in the horizontal and verticaldirections to the middle silicon layer located between the frame and themass in order to form free-moving resilient beams and connection beams;

Step 6, corrode the exposed silicon dioxide layer;

Step 7, perform an one-step bonding to the top cap silicon wafer, theprocessed double side SOI silicon wafer, and the bottom cap siliconwafer.

According to the present invention, the fabrication technique for theaccelerometer further includes the following steps:

The fabrication technique for the top cap silicon wafer and bottom capsilicon wafer further comprises:

A. use photolithography, deep etching and etching to form a through holeon the top cap silicon wafer or the bottom cap silicon wafer;

B. use photolithography, deep etching and etching to respectively form arecess area on each bonding surface of the top cap silicon wafer andbottom cap silicon wafer;

C. clean the top cap wafer and bottom cap wafer before bonding with thedouble side SOI silicon wafer;

D. after bonding with the double side SOI silicon wafer, deposit metalon the surfaces of top cap silicon wafer and bottom cap silicon wafer toform electrodes; and deposit metal on the surface of double side SOIsilicon wafer through the through hole formed on the top cap siliconwafer or bottom cap silicon wafer to form an electrode from the throughhole.

In the fabrication technique disclosed in the present invention, thesilicon dioxide layers serve the purpose of protecting the siliconlayers, which are covered up by the silicon dioxide layers, from etchingor corrosion.

The said depth etching and etching method includes one or more from thefollowing methods: dry etching or wet etching; the said dry etchingcomprises: silicon deep reactive-ion etching and reactive-ion etching.

The etchant for etching the silicon layer comprises one kind or acombination of the following etchants: potassium hydroxide,tetramethylammonium hydroxide, ethylenediamine pyrocatechol or gaseousxenon difluoride.

The etchant for etching the silicon dioxide layer comprises one kind ora combination of the following etchants: buffered hydrofluoric acid, 49%hydrofluoric acid or gaseous hydrogen fluoride. The said silicon dioxidelayer can also be removed by reactive-ion etching of the dry etchingmethod.

In accordance to the present invention, the accelerometer and itsfabrication technique has the following advantages. First of all, thearrangement of two layers of symmetrical resilient folding beams betweenthe mass and the support frame makes the overall structure moresymmetrical and stable. The magnitude of displacement of each set ofresilient beam is relatively small under condition of acceleration.Furthermore, by adding a connection beam between two resilient foldingbeams, the invention further limits the magnitude of displacement ofeach set of resilient beam. Thus it makes the accelerometer'svibrational modes completely symmetrical with respect to the center ofthe mass; and the accelerometer is able to detect the slightestacceleration. Also, the present invention has a large mode isolationratio, and the mass is symmetrical in high order vibrational modes; theresonance frequency of the resilient beam is much higher than thebaseband frequency, which further decreases the noise of the MEMS chip.Therefore, the accelerometer has the advantages of high measurementaccuracy, high stability, and low noise.

The fabrication technique disclosed in the present invention uses doubleside SOI silicon wafer and one-step bonding technique for three piecesof silicon wafer; by etching the resilient beams and the mass on thedouble side SOI silicon wafer, the components can be accurately alignedand forms a highly symmetrical structure. Compared with the prior art,which uses bonding technique to fabricate the dual-layer beams, theaccelerometer fabricated by the present technique has high accuracy andsmall error; the throughput yield is also significantly increased. Sincethe etching technique is relatively simple, the present fabricationtechnique also has a high productive efficiency and low manufacturingcost.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a structure scheme of the present invention;

FIG. 2 is a perspective view of the measurement mass in the presentinvention;

FIG. 3 is a top view of the measurement mass in the present invention;

FIG. 4 is a diagram of step 1 to step 3 of the fabrication technique inaccordance with the present invention;

FIG. 5 is a diagram of step 4 to step 6 of the fabrication technique inaccordance with the present invention;

FIG. 6 is a diagram of step 7 of the fabrication technique in accordancewith the present invention.

DETAILED DESCRIPTION

The present invention will be described in further detail below withreference to the drawings and specific embodiments.

With reference to FIG. 1, an accelerometer comprises: measurement mass1, top cap silicon wafer 2 and bottom cap silicon wafer 3, which bothare coupled with the measurement mass 1; electrodes 4 are respectivelyprovided on the measurement mass 1, the top cap silicon wafer 2, and thebottom cap silicon wafer 3. The measurement mass 1 has a double side SOIstructure, it comprises top silicon layer 5, middle silicon layer 6, andbottom silicon layer 7; silicon dioxide layers 8 are provided betweenevery two silicon layers. A plurality of resilient beams 12 arerespectively formed in the top silicon layer 5 and the bottom siliconlayer 7.

With reference to FIGS. 2 and 3, the measurement mass 1 comprisessupport frame 11, a plurality of resilient beams 12, and a mass 13. Theresilient beams 12 and the mass 13 are located within the support frame11; the mass 13 is coupled with the support frame 11 by multiple sets ofresilient beams 13; each set of resilient beam 12 includes two resilientfolding beams 121, which are symmetrically provided with respect to themidline of the mass 13. Each set of resilient folding beams 121 areconnected by a connection beam 122. The connection beam 122 is aresilient connection beam. The two ends of resilient folding beams arelocated along the same line.

With reference to FIGS. 2 and 3, preferably, the mass 13 is arectangular body, and its cross section is a square. Multiple sets ofresilient beams 12 are located in the top silicon layer 5 and bottomsilicon layer 7. Preferably, four sets of resilient beams 12 aresymmetrically provided in each silicon layer. Each set of resilient beam12 includes two resilient folding beams 121. One end of each resilientfolding beam 121 is connected to the corner 131 of the mass 13; thus itkeeps the mass 13 at a horizontal position when there is noacceleration. When the acceleration is detected, the resilient foldingbeams 121, which are provided at the corners 131, will limit the area ofmovement of the mass 13, and prevent from reading over-limit or errordata. The shape of the mass 13 in the present invention however is notlimited to a rectangular shape; it can also be other shapes, such ashexagon, octagon, circle, etc. The arrangement of resilient beams 12 isalso not limited to a two layer structure, where each layer containsfour groups of resilient beams 12; it can also be a structure containingmultiple layers and multiple groups.

With reference to FIGS. 1 and 3, the measurement mass 1 has a doubleside SOI structure, it comprises top silicon layer 5, middle siliconlayer 6, and bottom silicon layer 7; silicon dioxide layers 8 areprovided between every two silicon layers. Multiple resilient beams 12are respectively formed in the top silicon layer 5 and the bottomsilicon layer 7. Electrodes 4 are provided on the measurement mass 1,the top cap silicon wafer 2, and the bottom cap silicon wafer 3. Acapacitor is formed between the measurement mass 1, the top cap siliconwafer 2, and the bottom cap silicon wafer once the electric circuit isclosed. When there is no acceleration, the capacitance between themeasurement mass 1, the top cap silicon wafer 2, and the bottom capsilicon wafer is constant. When the acceleration is detected, thesupport frame 11 will displace towards the acceleration direction.Meanwhile, the resilient beams 12 will also have a certain amount ofdisplacement. However, due to inertia, the magnitude of displacement ofthe mass 13 is relatively small. According to equation C=εA/d, where thecapacitance between two parallel conductive plates equals to thepermittivity of material (ε) times area (A) divide by the separationdistance (d). When displacement is generated due to acceleration, thedistance between the mass 13, the top cap silicon wafer 2 and bottom capsilicon wafer 3 changes. Therefore, the capacitance between themeasurement mass 1, the top cap silicon wafer 2 and the bottom capsilicon wafer 3 also changes. The integrated circuit can calculate themeasured acceleration based on the change of capacitance. After theacceleration disappears, the resilient beams 12 will return to itsoriginal state, which returns the capacitance between the mass 13, thetop cap silicon wafer 2 and the bottom cap silicon wafer 3 to a constantvalue.

By providing two layers of resilient beams 12, which are symmetricalalong the midline of the mass 13, in between the mass 13 and the supportframe 11, the invention effectively reduces the magnitude ofdisplacement of each resilient folding beam 121. It also unifiesmagnitude of displacement of each resilient folding beam 121. By addingthe connection beam 122 between the resilient folding beams 121, theinvention further unifies the magnitude of displacement of eachresilient folding beam 121. Thus it decreases the possibility of readingover-limit or error data due to the over-limit displacement of theresilient beams 12. Furthermore, since the magnitude of each resilientfolding beam 121 is relatively small, the time required for eachresilient folding beam 121 to return to its original state has beenshorten. The arrangement of multiple resilient beams 12 also makes theaccelerometer be able to detect the slightest acceleration, andincreases the accelerometer's measurement accuracy.

Next, the fabrication technique for the accelerometer disclosed in thepresent invention is described with reference to FIG. 4, FIG. 5, andFIG. 6, which includes the following steps:

Step 1, coat a layer of photoresist on the surface of the top siliconlayer 5 and the bottom silicon layer 7 of the double side SOI siliconwafer. Then expose the top silicon layer 5 and the bottom silicon layer7 according to certain patterns, and develop the patterns withdevelopers to make the patterns apparent. After that, etch the exposedpart of the top silicon layer 5 and the bottom silicon layer 7 to thesilicon dioxide layer 8 by using the deep reactive ion etching. Theexposed part of the silicon dioxide layer 8 is further etched using thedry reactive ion etching or buffered hydrofluoric acid, which form aplurality of holes, which are deep to the middle silicon layer 6. Thephotoresist is removed afterwards.

Step 2, deposit polycrystalline silicon to fill up the holes to form anelectric circuit; grow a layer of silicon dioxide on the surfaces of thetop silicon layer 5 and the bottom silicon layer 7. Then polish thesurfaces of the top silicon layer 5 and the bottom silicon layer 7 bychemical or mechanical polishing technique in order to meet thesmoothness requirement.

Step 3, coat a layer of photoresist on the surface of the top siliconlayer 5 and the bottom silicon layer 7. Then expose the top siliconlayer 5 and the bottom silicon layer 7 according to certain patterns,and develop the patterns developers to make the patterns apparent. Firstetch the exposed part of the grown silicon dioxide layer using dryreactive ion etching or buffered hydrofluoric acid. Then etch the topsilicon layer 5 and the bottom silicon layer 7 to the silicon dioxidelayer 8 using deep reactive ion etching. Finally etch the exposed partof the silicon dioxide layer using dry reactive ion etching or bufferedhydrofluoric acid, to thereby form a plurality of resilient beams 12. Alayer of silicon dioxide is either grown or chemical vapor deposited(CVD) on top of the resilient beams 12.

Step 4, etch to remove the exposed part of the silicon dioxide layer 8by using dry etching method. Then deep etch the middle silicon layer 6to a certain depth by using silicon deep reactive ion etching or gaseousxenon difluoride.

Step 5, etch the middle silicon layer 6, which has been etched to acertain depth, in both horizontal and vertical directions by usingpotassium hydroxide, or tetramethylammonium hydroxide, orethylenediamine pyrocatechol, or gaseous xenon difluoride. The etchingtime is controlled based on the size of the region to be etched away inthe middle silicon layer 6. After the middle silicon layer 6 is etched,two layers of free-moving resilient beams 12 and connection beams 122are formed.

Step 6, etch to remove the silicon dioxide exposed on the siliconsurface by using buffered hydrofluoric acid, 49% hydrofluoric acid, orgaseous hydrogen fluoride.

Step 7, Perform an one-step-bonding to the top cap silicon wafer, theprocessed double side SOI silicon wafer and the bottom cap siliconwafer.

The fabrication technique of the accelerometer disclosed in the presentinvention further includes the following steps:

The fabrication technique for the top cap silicon wafer and bottom capsilicon wafer also includes:

A. before bonding with the double side SOI silicon wafer, coatphotoresist on the top cap silicon wafer 2 or the bottom cap siliconwafer 3; then expose according to certain patterns, and develop withdeveloper to make the patterns apparent. Etch the exposed part of thetop cap silicon wafer 2 or the bottom cap silicon wafer 3 until thesilicon dioxide layer 8 is exposed by using deep reactive ion etchingmethod, potassium hydroxide, tetramethyl ammonium hydroxide, orethylenediamine phosphorus hydroquinone. Then further etch the exposedportion of the silicon dioxide layer 8 to form a through hole by usingbuffered hydrofluoric acid, 49% hydrofluoric acid, or gaseous hydrogenfluoride. The photoresist is removed in the end.

B, coat photoresist on the top cap silicon wafer 2 and the bottom capsilicon wafer 3; then expose according to certain patterns, and developwith developers to make the patterns apparent. Respectively etch theexposed parts of the top cap silicon wafer 2 and the bottom cap siliconwafer 3 to a certain depth by using deep reactive ion etching method,potassium hydroxide, tetramethylammonium hydroxide, or ethylenediaminepyrocatechol, to thereby respectively form a recess area on each of thebonding surfaces of top cap silicon wafer 2 and bottom cap silicon wafer3.

C, clean the top cap silicon wafer 2 and the bottom cap silicon wafer 3before bonding with the double side SOI silicon layer.

D, after boding with the double side SOI silicon wafer, deposit metal onthe surfaces of the top cap silicon wafer 2 and bottom cap silicon wafer3 to form electrodes 4; deposit metal on the surface of double side SOIsilicon wafer through the through hole formed on either the top capsilicon wafer 3 or bottom cap silicon wafer 4 to form an electrode 4through the through hole.

The silicon dioxide layers mentioned in the above fabrication techniqueserve the purpose of protecting the silicon layers, which are covered upby the silicon dioxide layers, from etching or corrosion.

The deep etching method and etching method mentioned in the presentinvention includes one or more of the following methods: dry etching orwet etching. The said dry etching includes, silicon deep reactive-ionetching and reactive-ion etching.

The material, equipment, and techniques used in the present inventionhave been disclosed by prior arts, but the accelerometer is dramaticallyimproved by using these material and techniques, particularly by usingthe double side SOI silicon wafer. The resilient beams 12 and the mass13, formed by etching the double side SOI silicon wafer, are accuratelyaligned and form a highly symmetrical structure. Compared with the priorart, which fabricates the two level beams by bonding, the accelerometerfabricated using the present technique has higher accuracy and smallerror; the throughput yield is also increased. Since the etchingtechnique process is relatively simple, the present fabricationtechnique also has high production efficiency and low manufacturingcost. Furthermore, the geometry and vibration modes of the presentaccelerometer are symmetrical, which further increases the measurementaccuracy.

1. An accelerometer, comprising: a measurement mass; a top cap siliconwafer and a bottom cap silicon wafer, both coupled with the measurementmass; wherein the measurement mass includes a support frame, a mass, anda plurality of resilient beams; wherein the mass and the resilient beamsare located within the support frame, characterized in that the mass andthe support frame are connected by a plurality of sets of the resilientbeams, wherein each set comprises two resilient folding beams providedsymmetrically with respect to a midline of the mass; and a plurality ofconnection beam provided between each set of the resilient folding beamsto connect the resilient folding beams together.
 2. An accelerometeraccording to claim 1, wherein the connection beams are resilientconnection beams.
 3. An accelerometer according to claim 1, wherein eachresilient folding beam has two ends located along the same line.
 4. Anaccelerometer according to claim 1, wherein each resilient folding beamhas one end connected to a corner of the mass.
 5. An accelerometeraccording to claim 1, wherein the measurement mass has a double-sidedsilicon on insulator structure, which includes a top silicon layer, amiddle silicon layer, and a bottom silicon layer, with a silicon dioxidelayer provided between the top and middle silicon layers and between thebottom and middle silicon layers.
 6. An accelerometer according to claim5, wherein the resilient beams are symmetrically formed in both the topsilicon layer and the bottom silicon layer to form a double layerstructure.
 7. An accelerometer according to claim 1, wherein a firstelectrode is coupled with the measurement mass, a second electrode iscoupled with the top cap silicon wafer, and a third electrode is coupledwith the bottom cap silicon wafer.
 8. A method for fabricating anaccelerometer, comprising: (i) forming, by use of photolithography, deepetching and etching, a plurality of holes penetrating from a top siliconlayer and a bottom silicon layer to a middle silicon layer of adouble-sided silicon on insulator (SOI) silicon wafer; (ii) depositingpolycrystalline silicon in the holes to fill up the holes, then growinga silicon dioxide layer on surfaces of the top silicon layer and bottomsilicon layer of the double-sided SOI silicon wafer; (iii) forming, byuse of photolithography, deep etching and etching, a plurality of setsof symmetrical resilient folding beams and a plurality of connectionbeams for connecting the resilient folding beams, in the top siliconlayer and the bottom silicon layer of the double-sided SOI siliconwafer; then growing, by use of thermal oxidation, silicon dioxide onexposed surfaces of the resilient folding beams and the connectionbeams, or depositing, by use of chemical vapor deposition (CVD), a layerof silicon dioxide on exposed surfaces of the resilient folding beamsand the connection beams; (iv) removing, by use of photolithography andetching, the exposed silicon dioxide in the middle silicon layer, anddeep etching the middle silicon layer to a certain depth; (v) corrodingthe middle silicon layer located between a support frame and a mass inorder to form free-moving resilient beams and connection beams; (vi)removing by etching the exposed silicon dioxide; and (vii) bondingtogether in one-step the top cap silicon wafer, the processeddouble-sided SOI silicon wafer, and the bottom cap silicon wafer.
 9. Anaccelerometer fabrication technique according to claim 8, wherein, thefabrication technique for the top cap silicon wafer and bottom capsilicon wafer further comprises: A. forming, by use of photolithography,deep etching and etching, a through hole on the top cap silicon wafer orthe bottom cap silicon wafer; B. forming, by use of photolithography,deep etching and etching, a recess area on each bonding surface of thetop cap silicon wafer and bottom cap silicon wafer; C. before thebonding step, cleaning the top cap silicon wafer and bottom cap siliconwafer; D. after the bonding step, depositing metal on the surfaces oftop cap silicon wafer and bottom cap silicon wafer to form electrodes,and depositing metal on the surface of double-sided SOI silicon waferthrough the through hole formed on the top cap silicon wafer or thebottom cap silicon wafer to form an electrode from the through hole. 10.An accelerometer fabrication technique according to claim 8, wherein thedeep etching or etching method is selected from one or more of thefollowing methods: dry etching or wet etching; and the dry etchingcomprises silicon deep reactive ion etching and reactive ion etching.11. An accelerometer fabrication technique according to claim 9, whereinthe deep etching or etching method is selected from one or more of thefollowing methods: dry etching or wet etching; and the dry etchingcomprises silicon deep reactive ion etching and reactive ion etching.12. An accelerometer fabrication technique according to claim 8, whereinthe etchant for etching the silicon layer comprises one or more of thefollowing etchants: potassium hydroxide, tetramethylammonium hydroxide,ethylenediamine pyrocatechol or gaseous xenon difluoride.
 13. Anaccelerometer fabrication technique according to claim 8, wherein theetchant for etching the silicon dioxide layer comprises one or more ofthe following etchants: buffered hydrofluoric acid, 49% hydrofluoricacid or gaseous hydrogen fluoride.
 14. An accelerometer, comprising: ameasurement mass contained within a support frame and having a mass anda plurality of resilient beams, wherein the mass and the support frameare connected by a plurality of sets of the resilient beams, whereineach set comprises two resilient folding beams provided symmetricallywith respect to a midline of the mass; and a top cap silicon wafercoupled with the measurement mass; a bottom cap silicon wafer coupledwith the measurement mass; and a plurality of connection beams coupledbetween each set of the resilient folding beams.
 15. An accelerometeraccording to claim 14, wherein each resilient folding beam has two endslocated along the same line.
 16. An accelerometer according to claim 14,wherein each resilient folding beam has one end connected to a corner ofthe mass.
 17. An accelerometer according to claim 14, wherein themeasurement mass has a double-sided silicon on insulator structure,which includes a top silicon layer, a middle silicon layer, and a bottomsilicon layer, with a silicon dioxide layer provided between the top andmiddle silicon layers and between the bottom and middle silicon layers.18. An accelerometer according to claim 17, wherein the resilient beamsare symmetrically formed in both the top silicon layer and the bottomsilicon layer to form a double layer structure.
 19. An accelerometeraccording to claim 14, wherein a first electrode is coupled with themeasurement mass, a second electrode is coupled with the top cap siliconwafer, and a third electrode is coupled with the bottom cap siliconwafer.
 20. A method for fabricating an accelerometer, comprising: (i)forming a plurality of holes penetrating from a top silicon layer and abottom silicon layer to a middle silicon layer of a double-sided siliconon insulator (SOI) silicon wafer; (ii) depositing polycrystallinesilicon in the holes to fill up the holes, then growing a silicondioxide layer on surfaces of the top silicon layer and bottom siliconlayer of the double-sided SOI silicon wafer; (iii) forming a pluralityof sets of symmetrical resilient folding beams and a plurality ofconnection beams for connecting the resilient folding beams, in the topsilicon layer and the bottom silicon layer of the double-sided SOIsilicon wafer; then growing silicon dioxide on exposed surfaces of theresilient folding beams and the connection beams, or depositing a layerof silicon dioxide on exposed surfaces of the resilient folding beamsand the connection beams; (iv) removing the exposed silicon dioxide inthe middle silicon layer, and deep etching the middle silicon layer to acertain depth; (v) corroding the middle silicon layer located between asupport frame and a mass in order to form free-moving resilient beamsand connection beams; (vi) removing the exposed silicon dioxide; and(vii) bonding together in one-step the top cap silicon wafer, theprocessed double-sided SOI silicon wafer, and the bottom cap siliconwafer.